1. Field of the Invention
This invention is in the field of methods and apparatus for the application of coatings to glass containers. More particularly, the present invention is in the field of methods and devices for the application of coatings of varying thicknesses to bottles, jars and the like, where the distance between the closure region and the shoulder of the container is minimal.
2. Description of the Prior Art
The utility of glass bottles and jars has been broadened by surface coating to decrease abrasion and breakage, as taught by Carl, et al., U.S. Pat. No. 3,323,889; Gatchet, et al., U.S. Pat. No. 3,516, 811; Scholes, et al., U.S. Pat. No. 3,819,404; Hofmann, et al., U.S. Pat. No. 4,431,692; Lindner, et al., U.S. Pat. No. 4,668,268, and others. Gatchet observed the utility of avoiding all coating on the closure region of the container, known in the art as the "finish" region of the container. In U.S. Pat. No. 4,431,692, Hofmann taught maintaining the finish out of contact with the treatment gas. Several of the prior workers in this field have recognized the existence of non-linear currents in the coating-precursor stream, including omnidirectional turbulent currents and upwardly-moving convection currents.
It is known in the art of glass manufacture that uncoated glass is generally unsuitable for handling in high-speed operations because its brittleness renders the surface susceptible to potentially catastrophic damage. Specifically, bottles and other glass containers made in large numbers are susceptible to breakage in the course of being transferred through various manufacturing steps, or during a subsequent filling operation.
In order to minimize the problems thus encountered, a number of treatments have been applied to the containers as they are manufactured. Such treatments include, e.g., spraying with lubricant such as a wax or fatty acid, and applying reactive coatings by chemical-vapor-deposition (CVD) or spray pyrolysis methods.
Treatment by CVD typically can involve propelling a vapor of metal-containing species onto the hot glass-container surface to produce a thin layer of metal oxide, typically stannic or titanic oxide. This metal-oxide layer anchors the waxy lubricant which is added after annealing. Without the metal-oxide layer, such waxy lubricants do not adhere well to glass under the conditions encountered in a wet filling line.
In U.S. Pat. No. 4,668,268, assigned to the same assignee as the present invention, Lindner et al. teach the application of a metal-containing compound, generally an organotin material, to the surface of a glass container immediately after that container has acquired sufficient mechanical integrity to maintain its shape on a material-handling line. The Lindner et al. disclosure describes a coating hood for applying a uniform protective coating to a glass container as the container is transported by material-handling means, generally a conveyor belt, after its formation from molten glass. The described coating hood comprises a pair of side walls with a coating jet in at least one of the walls, and an exhaust system to remove the process stream from the coating zone.
In the field relating to the coating of containers generally and glass bottles specifically, one problem which is repeatedly encountered is that of applying adequate material to the body of the container while keeping the area near the open end of the container untreated. The open end has a structure to accommodate a closure such as a cap with screw threads or a gasketed lid for a friction or vacuum seal. This portion of the container is referred to in the art as the "finish" of the bottle or jar.
It is desirable to maintain the finish portion of the container relatively free of coating material for reasons of both chemical and physical importance. Where an improperly oxidized tin compound is deposited on the screw threads of, for instance, a jar for holding baby food, the chemical, electrochemical or mechanical interaction between the metal cap and the coating may be sufficient to discolor the glass or corrode the metal. Another disadvantage of coating on the finish is the possible effect on the frictional interaction between the glass and the cap or other closure; low friction can permit leakage, while too high a frictional value can impede both placement and removal of the closure. In either case, the utility of the treated container is adversely affected.
In the art of coating glass containers, the film deposited onto the glass surface is measured in arbitrary coating-thickness units (CTU), the unit thickness being about 2.5 Angstroms (.ANG.); tin-oxide coatings of from about 30 to 40 CTU's, or about 75 to 100 .ANG., may be required for the body of the container, while acceptable coating on the finish may be one-half or even one-tenth of this amount, depending upon the ware and its intended use. While manufacturers of baby food state a preference for finish coating of less than half the shoulder coating, proximity of finish to shoulder has heretofore made the desired separation difficult or impossible to achieve under the teachings of the prior art.
The improvement in the art which Lindner et al. provided in partial response to the problem of differential wall and finish thickness was accomplished by directing a stream of air, in which no coating material was entrained, onto the finish in order to displace and dilute coating material which would otherwise coat the finish region almost as much as the shoulder or the body of the container. By selection of the geometry of the coating hood as a function of the containers to be coated, Lindner et al. were able to effect acceptable coating thickness on the sidewalls, concomitant with protection of the finish for the large volume of glass containers having necks of appreciable length.
However, the utility of the apparatus of the Lindner et al. patent is only marginal for applications such as food and cosmetic bottles and jars having very short or non-existent necks. The latter group comprises, e.g., jam, jelly and cold-cream jars, and containers for baby foods, peanut butter, thixotropic salad dressings, and the like.
While the most effective prior art directs the vapor-laden air stream horizontally at the label panel of the ware, and a stream of vapor-free air at the finish region, some of the coating stream has been found inevitably to be displaced toward the finish by conditions within the coating hood. Displacement can originate, e.g., in the shearing interaction between adjacent coating streams traversing the hood in opposite directions; in the turbulence caused by the ware as it crosses the coating streams; in the strong convection currents caused by hot ware moving through a coating stream that is typically hundreds of degrees cooler than the ware; and in the induced draft caused by the finish-protection stream.
An objective of the current invention is minimization of the vertical displacement of the essentially horizontal coating stream. A further objective is utilization of this essentially horizontal coating stream to provide adequate coating for baby-food jars and other substantially non- or short-necked ware, typically defined to include a coating thickness on the finish of less than half the coating thickness on the sidewall.
The objective is achieved by separate coating zones for opposing sides of the ware, to minimize shearing interaction; and by two or more finish-protection (i.e., non-treating) process streams, or fresh air, traveling above and essentially parallel with the two or more coating streams, the finish-protection streams having sufficient velocity to dilute and displace any coating vapors that may be approaching the finish because of the convection or turbulence mentioned previously, the non-treating process stream having insufficient velocity to cause induced drafts which are known, in the prior art, to bring coating vapors into the finish zone.